The insulation material industry has determined that it is desirable to utilize fibers in heat and sound insulating applications which are not durable in physiological fluids, such as lung fluid. While candidate materials have been proposed, the use temperature limit of these materials have not been high enough to accommodate many of the applications to which high temperature resistant fibers, including refractory glass and ceramic fibers, are applied. In particular, high temperature resistant fibers should exhibit minimal linear shrinkage at expected exposure temperatures, in order to provide effective thermal protection to the article being insulated.
Many compositions within the man-made vitreous fiber family of materials have been proposed which are decomposable in a physiological medium. These glass fibers generally have a significant alkali metal oxide content, which often results in a low use temperature limit.
Canadian Patent Application 2017344 describes a glass fiber having physiological solubility formed from glasses containing as required components silica, calcia and Na.sub.2 O, as preferred components, magnesia and K.sub.2 O, and as optional components boria, alumina, titania, iron oxides, and fluoride.
International Publication WO 90/02713 describes mineral fibers which are soluble in saline solutions, the fibers having a composition including silica, alumina, iron oxide, calcia, magnesia, Na.sub.2 O and K.sub.2 O.
U.S. Pat. No. 5,108,957 describes glass compositions useful for forming fibers which are able to be degraded in a physiological medium containing as required components silica, calcia, Na.sub.2 O plus K.sub.2 O, and boria, and optionally alumina, magnesia, fluoride and P.sub.2 O.sub.5. It describes the presence of phosphorus as having the effect of increasing the rate of decomposition of the fibers in a physiological medium.
Other patents which cite the effect of phosphorus in favoring biological solubility of mineral fibers include International Publication WO 92/09536, describing mineral fibers containing substantially silica and calcia, but optionally magnesia and Na.sub.2 O plus K.sub.2 O, in which the presence of phosphorus oxide decreases the stabilizing effect of aluminum and iron on the glass matrix. These fibers are typically produced at lower temperatures than refractory ceramic fibers. We have observed that at melt temperatures required for high temperature resistant fibers (1700-2000.degree. C.), phosphorus oxide at levels as low as a few percent can cause severe degradation and/or erosion of furnace components.
Canadian Patent Application 2043699 describes fibers which decompose in the presence of a physiological medium, which contain silica, alumina, calcia, magnesia, P.sub.2 O.sub.5, optionally iron oxide, and Na.sub.2 O plus K.sub.2 O. French Patent Application 2662687 describe mineral fibers which decompose in the presence of a physiological medium, which contain silica, alumina, calcia, magnesia, P.sub.2 O.sub.5, iron oxide and Na.sub.2 O plus K.sub.2 O plus TiO.sub.2.
U.S. Pat. No. 4,604,097 describes a bioabsorbable glass fiber comprising generally a binary mixture of calcia and phosphorous pentoxide, but having other constituents such as calcium fluoride, water, and one or more oxides such as magnesia, zinc oxide, strontium oxide, sodium oxide, potassium oxide, lithium oxide or aluminum oxide.
International Publication WO 92/07801 describes a bioabsorbable glass fiber comprising phosphorous pentoxide, and iron oxide. A portion of the P.sub.2 O.sub.5 may be replaced by silica, and a portion of the iron oxide may be replaced by alumina. Optionally the fiber contains a divalent cation compound selected from Ca, Zn and/or Mg, and an alkali metal cation compound selected from Na, K, and/or Li.
U.S. Pat. No. 5,055,428 describes a soda lime aluminoboro-silicate glass fiber composition which is soluble in a synthetic lung solution. Alumina content is decreased with an increase in boria, and an adjustment in silica, calcia, magnesia, K.sub.2 O and optionally Na.sub.2 O. Other components may include iron oxide, titania, fluorine, barium oxide and zinc oxide.
International Publication WO 87/05007 describes inorganic fiber having solubility in saline solution and including silica, calcia, magnesia, and optionally alumina. International Publication WO 89/12032 describes inorganic fiber having extractable silicon in physiological saline solution and including silica, calcia, optionally magnesia, alkali metal oxides, and one or more of alumina, zirconia, titania, boria and iron oxides.
International Publication WO 93/15028 describes vitreous fibers that are saline soluble which in one usage crystallize to diopside upon exposure to 1000.degree. C. and/or 800.degree. C. for 24 hours and have the composition described in weight percent of silica 59-64, alumnina 0-3.5, calcia 19-23 and magnesia 14-17, and which in another usage crystallize to wollastonite/pseudowollastonite and have the composition described in weight percent of silica 60-67, alumina 0-3.5, calcia 26-35 and magnesia 4-6.
The fibers described in the above identified patent publications are limited, however, in their use temperature, and are therefore unsuitable for high temperature insulation applications, such as furnace linings for use above 1000.degree. C., and reinforcement applications such as metal matrix composites and friction applications.
A product based on non-durable fiber chemistry is marketed by Unifrax Corporation (Niagara Falls, N.Y.) under the trademark INSULFRAX, having the nominal weight percent composition of 65% SiO.sub.2, 31.1% CaO, 3.2% MgO, 0.3% Al.sub.2 O.sub.3 and 0.3% Fe.sub.2 O.sub.3. Another product is sold by Thermal Ceramics (located in Augusta, Ga.) under the trademark SUPERWOOL, and is composed of 58.5% SiO.sub.2, 35.4% CaO, 4.1% MgO and 0.7% Al.sub.2 O.sub.3 by weight. This material has a use limit of 1000.degree. C. and melts at approximately 1280.degree. C., which is too low to be desirable for the high temperature insulation purposes described above.
International Application WO 94/15883 discloses CaO/MgO/SiO.sub.2 fibers with additional constituents Al.sub.2 O.sub.3, ZrO.sub.2, and TiO.sub.2, for which saline solubility and refractoriness were investigated. That document states that saline solubility appeared to increase with increasing amounts of MgO, whereas ZrO.sub.2 and Al.sub.2 O.sub.3 were detrimental to solubility. The presence of TiO.sub.2 (0.71-0.74 mol %) and Al.sub.2 O.sub.3 (0.51-0.55 mol %) led to the fibers failing the shrinkage criterion of 3.5% or less at 1260.degree. C. The document further states that fibers that are too high in SiO.sub.2 are difficult or impossible to form, and cites samples having 70.04, 73.28 and 78.07% SiO.sub.2 as examples which could not be fiberized.
In addition to temperature resistance as expressed by shrinkage characteristics that are important in fibers that are used in insulation, it is also required that the fibers have mechanical strength characteristics during and following exposure to the use or service temperature, that will permit the fiber to maintain its structural integrity and insulating characteristics in use.
One characteristic of the mechanical integrity of a fiber is its after service friability. The more friable a fiber, that is, the more easily it is crushed or crumbled to a powder, the less mechanical integrity it possesses. We have observed that, in general, refractory fibers that exhibit both high temperature resistance and non-durability in physiological fluids also exhibit a high degree of after service friability. This results in the fiber's lacking the strength or mechanical integrity after exposure to the service temperature to be able to provide the necessary structure to accomplish its insulating purpose.
We have found that high temperature resistant, non-durable fibers which do exhibit good mechanical integrity have very small-scale or fine crystalline microstructures after exposure to service temperature. Other measures of mechanical integrity of fibers include compression strength and compression recovery.
Refractory glass compositions which may exhibit target durability, shrinkage at temperature, and strength characteristics may not, however, be susceptible to fiberization, either by spinning or blowing from a melt of its components.
It is therefore an object of the present invention to provide high temperature resistant refractory glass fiber, that is readily manufacturable from a melt having a viscosity suitable for blowing or spinning fiber, and which is non-durable in physiological fluids.
It is a further object of the present invention to provide high temperature resistant refractory glass fiber, which is non-durable in physiological fluids, and which exhibits good mechanical strength after exposure to service temperature.
It is a further object of the present invention to provide high temperature resistant refractory glass fiber, which is non-durable in physiological fluids, yet which exhibits low after service friability.
It is a further object of the present invention to provide high temperature resistant refractory glass fiber, which is non-durable in physiological fluids, and which preferably exhibits a small-scale or fine microstructure after initial exposure to service temperature.
It is a further object of the present invention to provide high temperature resistant refractory glass fiber, which is non-durable in physiological fluids, and which exhibits high compression strength and recovery from compression after exposure to service temperature.
It is a further object of the present invention to provide a high temperature resistant refractory glass fiber, which is non-durable in physiological fluids, and which exhibits low shrinkage at the use temperature.